You use them, and once you beat the challenge/reach the needed rank/get enough cash, you get the hero key back. With 5-6 hero keys, you can have quite a few heroes for free. Sounds like a good deal to me. Also makes me think we should expect quite a few more heroes in the future.
I was pleasantly surprised to find out they’d be refundable! I don’t get a lot of playtime and am really slow at unlocking characters, so it’s super-cool to hear that I don’t have to be hesitant to spend one on an easy unlock that I just haven’t gotten yet, since when I do get it I’ll get the token back. This leaves me feeling a lot less like I should save all my tokens for new heroes/high-rank or PvP-only unlocks that will take me a really long time to get through normal play. I’m pretty excited about the way this is gonna work.